It has been known for many years that silver is a useful antibacterial agent with broad-spectrum activity together with compatibility with mammalian tissue, and there have been many proposals to incorporate silver into wound dressings to obtain the advantage of the bactericidal properties of silver in a wound dressing. In addition, silver has been applied to fibrous material previously for non-wound dressing purposes, usually for the purpose of enhancing electrical conductivity. Silver has been applied to such fibers, which are generally not gel-forming, in a variety of ways some of which involve immersing the fibers into a silver solution but detail of the procedures used is often lacking.
Carboxymethyl cellulose, in particular carboxymethylated lyocell, has the ability to absorb a great deal of exudates or wound fluid and to form a gel on its surface. This property of the material has been found to be particularly advantageous in the formation of wound dressings that are both absorbent and gel-forming. The carboxymethylation of cellulose is described in WO93/12275 and the use of carboxymethyl cellulose for wound dressings is described in WO94/16746. Calcium (or sodium/calcium) alginate is another material useful in the formation of wound dressings, because of its absorbency and gelling capability. Gel-forming fibers for use in wound dressings are water-absorbent fibers which become moist and slippery or gelatinous upon the uptake of wound exudate and thus reduce the tendency for the fibers to adhere to the wound. The gel-forming fibers may also swell. Gel-forming fibers can be of the type which retain their structural integrity on absorption of exudate or can be of the type which lose their fibrous form and become a structureless gel or solution on absorption of exudate.
There have, however been particular problems with the use of silver in wound dressings because of the fact that silver compounds are light-sensitive and darken on exposure to light. This can result in the production of products which have an unattractive visual aspect, even if they are technically suitable for use as wound dressings.
There are three particular aspects of the darkening of silver compounds in light which need to be addressed when seeking to produce a commercially acceptable silvered wound dressing. One aspect is the actual color of the product, namely the desire to have a product having a color acceptable to the consumer. The second aspect is the desire to produce a product having a uniform appearance. The third aspect is the stability (shelf life) of the color of the dressing within its packaging.
In the past it has been proposed to apply silver to fibers by a process including the step of contacting the fibers with a solution containing silver ions under conditions which do not cause irreversible gelling of the fibers by contacting the fibers essentially simultaneously with an entire solution containing silver ions. Rapid immersion of the fibers in this way is said to provide a very uniform uptake of silver ions. Such a process is described in U.S. Publication No. 20040241213. However, the immersion takes place in an organic solvent to prevent irreversible gelling of the fibers. Not only might this limit the solvents that can be used, the use of organic solvents might raise environmental and cost issues. The production of fabric from silvered fibers limits the product forms available for use as wound dressings to certain types of fabrics.
In order to treat certain types of wounds, for example burns or surgical wounds, it is desirable to use a fabric, either woven or non-woven, in order to provide improved properties to the resulting dressing. Such properties can include wet tensile strength, flexibility, reduced brittleness and reduced shrinkage.
It would be advantageous to have available a process for applying silver to a fabric comprising gel-forming fibers rather than the prior art process for producing silvered fibers and then forming a fabric from them. The process would enable wound dressings with improved properties to be produced which would bring the benefits of silver to more types of wound. It would also be advantageous to have available a process that eliminates the use of an organic solvent in the application of silver.
Moreover, it would be advantageous to have available a process for producing a hydroentanged non-woven silvered fabric for use as a wound dressing and in particular one comprising hydroentanged carboxymethylcellulose fibers. One possible route to producing such a fabric would be to hydroentangle a cellulosic fabric which is then carboxymethylated and reacted with silver to give it antibacterial properties. Because the fabric is preformed it is not possible to randomize the fibers post treatment with silver. The process for reacting the fabric with silver therefore needs to give a uniform application of silver to the fabric.
Surprisingly, we have found that silver may be applied to a fabric made of gel-forming fibers by spraying the fabric with a silver solution.
The processes of the prior art shy away from spraying as the silver solution used to deliver silver ions to the fibers is an aqueous organic solution, especially an aqueous alcoholic solution such as a mixture of ethyl alcohol and water. The solvent is considered necessary to avoid irreversible gelling of the fibers. Generally, the spraying of alcoholic solutions is avoided because of flammability and toxicity issues and the problems they raise in ensuring the safety of the operatives engaged in the process.
Surprisingly, we have found that it is possible to reduce the level of solvent in the silver solution and even to eliminate it by spraying the fabric with an aqueous silver solution.